Lithium ion secondary batteries are secondary batteries having a high charge/discharge capacity and capable of achieving high output. Currently, lithium ion secondary batteries are mainly used as power supplies and the like for portable electronic equipment, and are expected to be used as power supplies for electric vehicles assumed to be used widely in the future. Lithium ion secondary batteries have, respectively in a positive electrode and a negative electrode, active materials capable of inserting and eliminating lithium (Li) ions therein/therefrom. The lithium ion secondary batteries operate when lithium ions move through an electrolytic solution provided between the two electrodes.
In lithium ion secondary batteries, a lithium-containing metallic complex oxide such as a lithium cobalt complex oxide is mainly used as the active material for the positive electrode, and a carbon material having a multilayer structure is mainly used as the active material for the negative electrode. The performance of a lithium ion secondary battery is influenced by materials of the electrolyte and the positive and negative electrodes included in the secondary battery. Research and development are actively conducted for a substance for the active materials. For example, usage of silicon or silicon oxide having a higher capacity (capable of inserting and eliminating a large amount of lithium ions) than carbon is discussed as a substance for the negative electrode active material.
When silicon is used as the negative electrode active material, a battery with a capacity higher than when a carbon material is used is obtained. However, silicon undergoes a large volume change associated with occlusion and release of Li during charging and discharging. Since silicon turns into a fine powder and becomes eliminated or detached from a current collector as a result, a battery using silicon as the negative electrode active material has a problem regarding having a short charge/discharge cycle life. Thus, by using a silicon oxide as the negative electrode active material, the volume change of the negative electrode active material associated with occlusion and release of Li during charging and discharging is suppressed better than when silicon is used.
For example, usage of a silicon oxide (SiOx: x is about 0.5≤x≤1.5) is discussed as the negative electrode active material. SiOx, when being heated, is known to decompose into Si and SiO2. This is referred to as a disproportionation reaction in which a solid separates into two phases, i.e., Si phase and SiO2 phase, through an internal reaction. The Si phase obtained from the separation is extremely fine. In addition, the SiO2 phase that covers the Si phase has a function of suppressing decomposition of the electrolytic solution. Thus, the secondary battery using the negative electrode active material formed of SiOx that has been decomposed into Si and SiO2 has excellent cycle characteristics.
The cycle characteristics of the secondary battery improve further when the secondary battery uses, as the negative electrode active material, finer silicon particles forming the Si phase of the SiOx described above. JP3865033 (B2) (Patent Literature 1) discloses a method of heating metal silicon and SiO2 to sublimate those into a silicon oxide gas, and cooling the gas to produce SiOx. With this method, the particle size of the silicon particles forming the Si phase can be set to a nano size level of 1-5 nm.
JP2009102219 (A) (Patent Literature 2) discloses a production method including decomposing a silicon raw material into an elemental state in a high temperature plasma, rapidly cooling that to the temperature of liquid nitrogen to obtain silicon nano particles, and fixing the silicon nano particles into a SiO2—TiO2 matrix by using a sol-gel method or the like.
In the production method according to Patent Literature 1, the matrix is limited to those that are sublimatable. In addition, in the production method according to Patent Literature 2, high energy for plasma discharge is required. Furthermore, silicon complexes obtained from these production methods have a flaw regarding the silicon particles of the Si phase having low dispersibility and being easily aggregated. When the Si particles aggregate with each other and the particle size thereof becomes large, the secondary battery using that as the negative electrode active material results in having a low initial capacity and deteriorated cycle characteristics.
In recent years, nano silicon materials that are expected for usage in semiconductors, electrics or electronics fields, and the like have been developed. For example, Physical Review B (1993), vol. 48, 8172-8189 (Non-Patent Literature 1) discloses a method for synthesizing a layered polysilane by causing a reaction between hydrogen chloride (HCl) and calcium disilicide (CaSi2), and describes that the layered polysilane obtained in this manner can be used in a light-emitting element or the like.
JP2011090806 (A) (Patent Literature 3) discloses a lithium ion secondary battery using a layered polysilane as the negative electrode active material.